This nonprovisional application claims priority under 35 U.S.C. xc2xa7119(a) on Patent Application No. 2001-224282 filed in Japan on Jul. 25, 2001, the entirety of which is herein incorporated by reference.
The present invention relates to a starting device and a method for starting an internal combustion engine provided with a crankshaft to be rotated by an electric motor at startup with the starting device, and more particularly to a starting device having an electric motor and a decompression mechanism for opening an engine valve which is lifted by a prescribed amount to reduce the compression pressure during the compression stroke of the internal combustion engine.
Internal combustion engines having a crankshaft rotated by a starter motor during startup are well known. The internal combustion engine having a decompression mechanism for opening the engine valve to be opened and closed by a valve train cam provided on the camshaft that is rotated synchronously with the rotation of the crankshaft is also known.
For example, in Japanese Patent Document 70366/1994, a decompression unit having a decompression cam and a reversing decompression cam supported on the camshaft via a one-way clutch is described. In the case where a piston in the compression stroke is moved slightly backward by the compression pressure when the internal combustion engine is stopped, the camshaft rotates in the reverse direction. The reversing decompression cam rotates integrally with the camshaft by action of the one-way clutch and opens an exhaust valve to decrease the compression pressure in a combustion chamber at the next startup of the engine.
When reverse rotation of the camshaft does not occur when the internal combustion engine is stopped, e.g., when the piston is in the expansion stroke, the decompression cam opens the exhaust valve during the compression stroke after the next startup timing to reduce the compression pressure in the combustion chamber. With such a decompression unit, decompression operation for reducing the compression pressure is performed only in the first compression stroke after startup.
The present inventors have determined that the background art suffers from the following disadvantages. During startup of an internal combustion engine, the camshaft starts to rotate in the normal direction from a position where the camshaft stopped previously in the decompression unit of the background art. The crank angle from the position when the crankshaft starts to rotate in the normal direction to the point where the first compression stroke starts after stoppage of decompression operation (compression bottom dead center) (hereinafter referred to as xe2x80x9crun-up anglexe2x80x9d) is determined by the position where the camshaft stops when the internal combustion engine is stopped. Therefore, depending on the stopped positions, a sufficient run-up angle may not be secured.
Accordingly, the revolving speed (angular speed) of the crankshaft is not sufficient for the piston to get over the first compression top dead center after cease of decompression operation, thereby hindering smooth starting. Such a circumstance tends to occur especially when the sliding friction of the internal combustion engine is excessive, e.g., for example, in case of low temperature starts or the like.
Therefore, in order to ensure that the piston can get over the first compression top dead center, the generated driving torque must be increased in the case where the starter motor is used for starting the internal combustion engine. Accordingly, the starter motor may have to be upsized disadvantageously. In addition, with the decompression units in the background art, it is difficult to increase the run-up angle significantly because the decompression operation is performed only during the first compression stroke after startup. The present invention overcomes these shortcomings associated with the background art and achieves other advantages not realized by the background art.
An object of the present invention is to provide a starting method and starting device for an internal combustion engine in which the run-up angle is increased so that the piston can easily overcome the first compression top dead center, e.g., particularly after decompression operations at startup have stopped, without increasing the size and capacity of the electric motor and/or starting device for rotating the crankshaft.
These and other objects are accomplished by a starting method for an internal combustion engine comprising the steps of rotating a crankshaft with an electric motor during an engine startup; opening an engine valve which is opened and closed by a valve train cam by a decompression mechanism, wherein the valve train cam is provided on a camshaft that is rotated synchronously with a rotation of the crankshaft, wherein the decompression mechanism includes a decompression cam provided on the camshaft in such a manner that the decompression cam is capable of rotating in the rotational range of the camshaft between a first stop position of the camshaft in a reverse rotational direction and a second stop position of the camshaft in a normal rotational direction and has a cam profile to bring the engine valve into an opened state at the first stop position and into a closed state at the second stop position; rotating the crankshaft in the reverse direction with the electric motor to rotate the decompression cam in the reverse direction to place the decompression cam in the first stop position at startup; rotating the crankshaft in the normal rotational direction with the electric motor to rotate the decompression cam in the normal rotational direction; and opening the engine valve by the decompression cam during either a compression stroke included within the range of a prescribed crank angle in which the crankshaft is rotated in the reverse direction by the electric motor or included within the range within a first compression stroke after a start of normal rotation of the decompression cam, or during the time period until the decompression cam reaches the second stop position.
These and other objects are further accomplished by a starting method for an internal combustion engine comprising the steps of rotating a crankshaft with an electric motor during an engine startup; opening an engine valve which is opened and closed by a valve train cam by a decompression mechanism, wherein the valve train cam is provided on a camshaft that is rotated synchronously with a rotation of the crankshaft, wherein the decompression mechanism includes a decompression cam provided on the camshaft in such a manner that the decompression cam is capable of rotating in the rotational range of the camshaft between a first stop position of the camshaft in a reverse rotational direction and a second stop position of the camshaft in a normal rotational direction and has a cam profile to bring the engine valve into an opened state at the first stop position and into a closed state at the second stop position; rotating the crankshaft in the reverse rotational direction with the electric motor to rotate the decompression cam in the reverse direction to place the decompression cam in the first stop position at startup; rotating the crankshaft in the normal rotational direction with the electric motor to rotate the decompression cam in the normal direction; and opening the engine valve with the decompression cam at a plurality of compression strokes during a period until the decompression cam reaches the second stop position.
These and other objects are further accomplished by a starting device for an internal combustion engine, wherein the starting device includes an electric motor for rotating a crankshaft during an engine startup, an engine valve with a valve train cam, a control device for controlling rotation of the crankshaft with the electric motor, and a decompression mechanism for opening the engine valve to be opened and closed by the valve train cam provided on a camshaft that is rotated synchronously with rotation of the crankshaft, the decompression mechanism comprising a reverse rotation stopper defining a first stop position; a normal rotation stopper defining a second stop position; a decompression cam rotatably mounted on the camshaft so as to be capable of rotating in a rotational range of the camshaft between the first stop position in a reverse rotational direction of the camshaft and the second stop position in a normal rotational direction of the camshaft; a decompression cam profile for opening the engine valve at the first stop position and closing the same at the second stop position; a torque transmission device transmitting reverse rotation torque from the camshaft to the decompression cam, the torque transmission device including a constrained state in which relative rotation between the camshaft and the decompression cam is constrained during a reverse rotation of the crankshaft, and an unconstrained state in which a drag torque is transmitted in the normal direction from the camshaft to the decompression cam by permitting a relative rotation between the camshaft and the decompression cam during a normal rotation of the crankshaft; and a rotation control device alternately preventing and permitting dragging of the decompression cam between the first stop position and the second stop position in the normal rotational direction.
According to a first aspect of the present invention, the crankshaft is rotated in the reverse direction by a prescribed crank angle by the electric motor and thus the decompression cam is rotated in the reverse direction and then in the normal direction at startup. When the crankshaft is rotated in the reverse direction, the engine valve is opened by rotating the decompression cam in the reverse direction and placing the same at the first stop position. Next, the decompression cam is rotated in the normal direction after the crankshaft starts to rotate in the normal direction. Then, decompression operation is performed during the compression stroke, e.g., either the compression stroke included in the range of the prescribed crank angle by which the crankshaft is rotated in the reverse direction or the first compression stroke after normal rotation of the decompression cam during the time period until the decompression cam reaches the second stop position.
Accordingly, the run-up angle increases by the extent of the prescribed crank angle by which the crankshaft is rotated in the reverse direction from the rotational position of the crankshaft at startup of the internal combustion engine. The revolving speed of the crankshaft at the first point of start of compression after stoppage of decompression operation thus increases, and the piston can easily overcome the first compression top dead center after stoppage of decompression operation. Therefore, the starting capability of the engine is improved without unnecessarily increasing the size and capacity of the electric motor that rotates the crankshaft. In addition, since the engine valve can always be opened at a certain position of the decompression cam when the crankshaft rotates in the normal direction by positioning, the angular range in which the engine valve can be opened by the decompression cam can be set to a certain range at each startup, thereby ensuring larger run-up angle than the related art.
According to a second aspect of the present invention, the crankshaft is rotated in the reverse direction by a prescribed crank angle by the electric motor and the decompression cam is rotated in the reverse direction and then in the normal direction at startup. Therefore, when the crankshaft is rotated in the reverse direction, the engine valve is opened by the decompression cam by rotating the decompression cam in the reverse direction and placing the decompression cam at the first stop position. The decompression cam is then rotated in the normal direction after the crankshaft starts to rotate in the normal direction. Decompression operation is then performed during a plurality of compression strokes until the decompression cam reaches the second stop position by rotating in the normal direction. Accordingly, decompression operation is performed during at least two compression strokes after the crankshaft starts rotating in the normal direction, and thus the run-up angle increases.
According to an additional aspect of the present invention, the following effects are exercised in addition to the effects described hereinabove. The torque transmission device includes the one-way clutch and the torque limiter provided in series in the torque transmission route from the camshaft to the decompression cam. When the crankshaft is further rotated in the reverse direction during which relative rotation between the camshaft and the decompression cam is disabled by the effect of the one-way clutch, the decompression cam abuts against the reverse rotation stopper and stopped at the first stop position by the torque limiter in a simple structure. The run-up angle increases correspondingly, and thus the revolving speed of the crankshaft at the first point of start of compression after stoppage of decompression operation increases. Accordingly, the piston can overcome the first compression top dead center after stoppage of decompression operation more easily. In addition, the torque limiter can prevent excessive torque from exerting on the decompression cam, the reverse rotation stopper, and the one-way clutch.
According to an additional aspect of the present invention, since the effective operation angle of the decompression cam is larger than the operation angle of the valve train cam which opens and closes the engine valve during the time that the valve is opened by the decompression cam at startup, decompression operation is not stopped by the first opening of the engine valve by the valve train cam after normal rotation has started. Instead, it is stopped at subsequent openings of the engine valve by the valve train cam. Accordingly, the advantageous effects of the present invention are obtained with a relatively simple structure depending on the configuration of the profile of the decompression cam. In the following description of the present invention, the various angles of operation and various angles are meant to be associated with the rotational angles of the crankshaft where otherwise not noted.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.